1. Field of the Invention
The present invention relates to ceramic electronic components. In particular, the present invention relates to an improvement in the structure and the material of a terminal portion of a ceramic electronic component, such as a monolithic ceramic capacitor, which includes a ceramic electronic component body (hereinafter referred to as a component body).
2. Description of the Related Art
When a ceramic electronic component such as a monolithic ceramic capacitor is mounted onto an aluminum board including an aluminum base having high heat dissipation and an insulating coating thereon, the ceramic electronic component readily breaks during thermal cycles including repeated temperature raising and lowering due to a large difference in thermal expansion coefficients between the aluminum board and the component body of the ceramic electronic component. In particular, a high-capacitance monolithic ceramic capacitor composed of a Pb-based ceramic dielectric material and used in electrical power supplies has a relatively low bending strength. Thus, the above problem is noticeable.
In order to solve this problem, a metal terminal member is soldered onto a terminal electrode of the ceramic electronic component so that the stress due to thermal expansion and shrinkage of a board is absorbed by the deformation or shift of the terminal member and is not directly applied to the component body.
FIG. 1 shows a ceramic electronic component 1 having the above structure. The ceramic electronic component 1 has a component body 2 and terminal members 3 and 4 attached at ends of the component body 2. The component body 2 is a rectangular parallelepiped having two end faces 5 and 6 opposing each other and four side faces 7, 8, 9 and 10 connecting these two end faces 5 and 6. Terminal electrodes 11 and 12 are formed on the end faces 5 and 6, respectively. The terminal electrodes 11 and 12 are formed by, for example, coating and baking a conductive paste, and extend over edge portions of the four side faces 7 to 10. The terminal members 3 and 4 are metal plates and are attached to the terminal electrodes 11 and 12, respectively, with solder.
FIG. 2 is a partial enlarged cross-sectional view at the side of the terminal member 3 of the ceramic electronic component 1 shown in FIG. 1. Since the structure at the side of the terminal member 4 is substantially the same as that of the terminal member 3 shown in FIG. 2, the following description is based on the side shown in FIG. 2.
Solder 13 bonds the terminal member 3 to the terminal electrode 11. In general, a high-temperature solder, such as a Pb-based solder, is used as the solder 13, so that the solder 13 is not softened or melted by heat during soldering the terminal member 3 to a conductive land on a mounting board (not shown in the drawing) when the ceramic electronic component 1 is mounted onto the mounting board.
Thus, soldering of the terminal member 3 and the terminal electrode 11 requires a relatively high temperature, and thus a relatively high thermal shock is applied to the terminal electrode 11 and the component body 2. The thermal shock causes stress in the terminal electrode 11, and a crack 15 will form in the component body 2 in some cases, as shown in FIG. 2. The likelihood of formation of the crack 15 is significant in a monolithic ceramic capacitor using the above Pb-based ceramic dielectric material.
The stress causing the crack 15 significantly affects the extended portion of the terminal electrode 11 on the side face 7. Thus, the crack 15 readily forms in the component body 2 in the vicinity of the edge of the extension of the terminal electrode 11.
This crack 15 causes decreased humidity resistance and decreased thermal shock resistance of the ceramic electronic component 1, and decreased electrical characteristics such as insulation resistance. Thus, the ceramic electronic component 1 is unreliable.
Such a crack 15 may be formed by future thermal shock due to a change in temperature of the ceramic electronic component 1, in addition to the thermal shock during soldering using the solder 13.
In FIG. 2, internal electrodes 16 and 17 formed in the component body 2 are shown. The component body 2 having the internal electrodes 16 and 17 functions as a monolithic ceramic capacitor. The internal electrodes 16 and the internal electrodes 17 are alternately arranged. The internal electrodes 16 are connected to the terminal electrode 11 whereas the internal electrodes 17 are connected to the terminal electrode 12 (see FIG. 1).
Some possible ideas to prevent the formation of the crack 15 include, for example, forming the terminal electrode 11 of a conductive resin containing metal powder and resin, or bonding the terminal member 3 to the terminal electrode 11 using a conductive resin as a bonding agent applied onto the terminal electrode 11, instead of the solder 13.
The conductive resin as the bonding agent used for bonding the terminal member 3 causes unsatisfactory appearance, such as contamination. Moreover, the conductive resin exhibits decreased shear strength at high temperatures, and is less reliable regarding bonding strength of the terminal member 3. Thus, the terminal member 3 may be detached from the component body 2 in some cases.
It is an object of the present invention to provide a ceramic electronic component not having the above problems.
A ceramic electronic component in accordance with the present invention includes at least one component body having two end faces opposing each other, side faces connecting the two end faces, and terminal electrodes formed on at least the end face, and terminal members, each including a metal plate and each being soldered to one of the terminal electrodes. Each of the terminal electrodes includes a metal layer formed only on the end face, a conductive resin layer formed on the metal layer, the conductive resin layer including a conductive resin containing metal powder and resin, and a plating film plated on the conductive resin layer.
In such a configuration, the plating film facilitates soldering between the terminal electrodes and the terminal members. Since the metal layer is formed only on the end face of the component body, stress which causes cracks in the component body is reduced during soldering and thermal shock. Moreover, the conductive resin layer relaxes the effect of the stress on the component body. Thus, the formation of cracks in the component body is prevented, and the ceramic electronic component is highly reliable regarding electrical characteristics.
In the present invention, the metal layer may be formed by coating and baking a conductive paste containing Ag, Agxe2x80x94Pd, Ni or Cu. In such a case, the metal film is thick and stress caused by soldering is increased. However, the above configuration can relax such an increased stress.
Preferably, the plating film includes a first plating film formed on the conductive resin layer and including a metal for preventing diffusion of a solder into the conductive resin layer, and a second plating film formed on the first plating film and including a metal having high solderability. Thus, deterioration of the conductive resin layer due to diffusion of the solder is prevented, and soldering is satisfactorily performed.
In the present invention, the terminal electrode has the conductive resin layer. Thus, a Snxe2x80x94Sb-based high-temperature solder having a high Young""s modulus can also be used instead of the Pb-based high-temperature solder. The use of the Pb-free solder is advantageous for environmental preservation.
The ceramic electronic component may include a plurality of component bodies, and each of the terminal members is commonly connected to one of the terminal electrodes of each of the component bodies.
Preferably, the component body forms a monolithic ceramic capacitor.